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United States Patent |
5,515,221
|
Gill
,   et al.
|
May 7, 1996
|
Magnetically stable shields for MR head
Abstract
First and second shield layers of a read head are constructed of a
lamination of NiMn and Fe-based layers to improve the performance of the
shield layers when they are subjected to high external fields, such as
from the pole tips of a write head combined therewith. Without lamination
with one or more NiMn layers, many shield materials do not return to the
same domain configuration after excitation from an external field. The
result is that the Fe-based material assumes a different domain
configuration after each excitation which changes the bias point of the MR
sensor of the read head. By laminating with NiMn, the uniaxial anisotropy
of the material can be increased to provide uniform domain configuration
and exchange pinning between shield material NiMn returns the material to
the same configuration after each external field excitation. The invention
further provides fine tunings of the magnetic properties of the shield
layer by various combinations of the Fe-based layers and/or the NiMn layer
with NiFe layers.
Inventors:
|
Gill; Hardayal S. (Portola Valley, CA);
Lin; Tsann (Saratoga, CA)
|
Assignee:
|
International Business Machines Corporation (Armonk, NY)
|
Appl. No.:
|
366940 |
Filed:
|
December 30, 1994 |
Current U.S. Class: |
360/319; 360/126; 360/317 |
Intern'l Class: |
G11B 005/127 |
Field of Search: |
360/113,126
|
References Cited
U.S. Patent Documents
4683012 | Jul., 1987 | Yamauchi et al. | 48/301.
|
4836865 | Jun., 1989 | Sakakima et al. | 148/306.
|
4843506 | Jun., 1989 | Gill et al. | 360/113.
|
5067991 | Nov., 1991 | Sawa et al. | 148/305.
|
5073214 | Dec., 1991 | Hirota et al. | 148/306.
|
5168409 | Dec., 1992 | Koyama et al. | 360/113.
|
5178689 | Jan., 1993 | Okamura et al. | 148/306.
|
5207841 | May., 1993 | Shigeta et al. | 148/307.
|
5208715 | May., 1993 | Mowry | 360/126.
|
5218497 | Jun., 1993 | Tanabe et al. | 360/113.
|
5264981 | Nov., 1993 | Campbell et al. | 360/126.
|
5287237 | Feb., 1994 | Kitada et al. | 360/113.
|
5436777 | Jul., 1995 | Soeya et al. | 360/113.
|
Foreign Patent Documents |
1-149941 | Jun., 1989 | JP.
| |
Primary Examiner: Wolff; John H.
Attorney, Agent or Firm: Baker, Maxham, Jester & Meador
Claims
We claim:
1. A combined magnetic head which includes an MR read head adjacent an
inductive write head comprising:
the inductive write head including:
a coil layer sandwiched between insulation layers, the insulation layers
being sandwiched between first and second pole piece layers; and
the first and second pole piece layers terminating in first and second pole
tips respectively at a head surface;
the MR read head including:
an MR sensor sandwiched between first and second gap layers;
first and second shield layers;
the first and second gap layers being sandwiched between the first and
second shield layers;
the MR sensor, the first and second gap layers and the first and second
shield layers terminating at said head surface;
the first and second shield layers being subjected to consecutive write
fields from the write head; and
at least one of the first and second shield layers being constructed of a
lamination of at least one antiferromagnetic layer and at least one soft
magnetic layer wherein the soft magnetic layer has an easy axis parallel
to the head surface and a hard axis perpendicular to the head surface, the
antiferromagnetic layer increasing magnetic domain stability of the soft
magnetic layer by increasing uniaxial anisotropy of the lamination,
whereby after the occurrence of each of a number of said consecutive write
fields, the said at least one of the first and second shield layers
returns to a substantially common magnetic domain state so that linear
response of the MR sensor remains substantially unchanged after each
consecutive write field.
2. A combined head as claimed in claim 1 wherein said antiferromagnetic
layer is comprised of NiMn.
3. A combined head as claimed in claim 2 including:
the lamination having an exchange-coupled field wherein the magnitude of
the exchange-coupled field is greater than the coercivity H.sub.C along an
easy axis of the shield layer.
4. A combined head as claimed in claim 2 including:
the lamination having a uniaxial anisotropy from 5 to 15 Oe, permeability
greater than 1,000, coercivity H.sub.C along an easy axis less than 3 Oe
and an exchange-coupled field between 3 Oe and 10 Oe.
5. A combined head as claimed in claim 2 including:
the soft magnetic shield layer being in direct contact with the
antiferromagnetic layer.
6. A combined head as claimed in claim 2 including:
the lamination further including a buffer layer sandwiched between the
antiferromagnetic layer and the soft magnetic layer.
7. A combined head as claimed in claim 6 including:
the buffer layer being NiFe with a thickness of 20 .ANG. to 200 .ANG..
8. A combined head as claimed in claim 2 wherein said soft magnetic layer
comprises an Fe alloy.
9. A combined head as claimed in claim 8 including:
the lamination further including a NiFe buffer layer being sandwiched
between the NiMn layer and the Fe alloy layer.
10. A combined head as claimed in claim 9 wherein said Fe alloy comprises
FeN.
11. A combined head as claimed in claim 10 including:
the FeN being 0.1-1.0 .mu.m thick; and
the buffer layer having a thickness of 20 .ANG. to 200 .ANG..
12. A combined head as claimed in claim 11 including:
the lamination includes a plurality of NiMn layers, a plurality of NiFe
buffer layers and a plurality of FeN layers laminated one after the other
in a sequential relationship.
13. A combined head as claimed in claim 8 including:
the Fe alloy layer comprises FeSiAl.
14. A combined head as claimed in claim 13 including:
the lamination further including a NiFe buffer layer sandwiched between the
NiMn layer and the FeSiAl layer.
15. A combined head as claimed in claim 14 including:
the NiFe buffer layer having a thickness of 20 .ANG. to 200 .ANG.;
another NiMn layer so that there are first and second NiMn layers; and
the FeSiAl layer being sandwiched between the first NiMn layer and the NiFe
buffer layer and the NiFe buffer layer being sandwiched between the FeSiAl
layer and the second NiMn layer.
16. A combined head as claimed in claim 8 including: the Fe alloy layer
being NiFe.
17. A combined head as claimed in claim 2 wherein said soft magnetic layer
is selected from the group consisting of NiFe, FeN and FeSiAl.
18. A combined magnetic head as claimed in claim 2 including:
means for moving magnetic media;
means trier supporting the magnetic head adjacent the moving magnetic
media; and
means connected to the magnetic head for processing signals from the
magnetic head.
19. A combined magnetic head as claimed in claim 2 wherein said first pole
layer and said first shield layer are a common layer.
20. A combined magnetic head as claimed in claim 2 wherein said first pole
layer and said first shield layer are separate layers.
21. A combined magnetic head as claimed in claim 1 including:
first and second shield layers, at least one of the shield layers being a
lamination;
said lamination comprising Set A and Set B;
Set A comprising one of a NiMn layer and a lamination of NiMn and a buffer
layer of NiFe; and
Set B comprising one of a Fe based layer and a lamination of an Fe based
layer and a buffer layer of NiFe.
22. A drive including the combined magnetic head of claim 21, the drive
including:
means for moving magnetic media;
means for supporting the magnetic head adjacent the moving magnetic media;
and
means connected to the magnetic head for processing signals from the
magnetic head.
23. A combined head as claimed in claim 21 including:
the lamination having a uniaxial anisotropy from 5 to 15 Oe, permeability
greater than 1000, coercivity H.sub.C along an easy axis less than 3 Oe
and an exchange field between 3 Oe and 10 Oe.
24. A combined head as claimed in claim 21 including:
each buffer layer being NiFe with a thickness of 20 .ANG. to 200 .ANG..
25. A combined head as claimed in claim 21 including:
a first Set A and a second Set A; and
Set B being sandwiched between said first Set A and said second Set A.
26. A combined head as claimed in claim 25 including:
the buffer layer being NiFe with a thickness of 20 .ANG. to 200 .ANG.; and
the Fe-based layer being 0.1-1 .mu.m thick.
27. A combined head as claimed in claim 26 including:
a plurality of laminations as claimed in claim 24.
28. A combined head including the lamination as claimed in claim 27, the
combined head including:
each of the first and second shield layers comprising said lamination.
29. A combined head as claimed in claim 28 including:
the first shield layer being common with a first pole piece of a write
head.
30. A combined magnetic head as claimed in claim 21 wherein said first pole
layer and said first shield layer are a common layer.
31. A combined magnetic head as claimed in claim 21 wherein said first pole
layer and said first shield layer are separate layers.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a novel lamination of materials which
provides a magnetically stable shield for an MR head and more particularly
to a shield which returns to a common state so that after perturbation by
a magnetic field, magnetostatic coupling between the shield and an MR
sensor remains stable.
2. Description of the Related Art
An MR head includes an MR sensor which is sandwiched between first and
second gap layers which are, in turn, sandwiched between first and second
shield layers. In a disk drive the MR head is mounted on a slider which is
connected to a suspension arm, the suspension arm urging the slider toward
a magnetic storage disk. When the disk is rotated the slider flies above
the surface of the disk on a cushion of air which is generated by the
rotating disk. The MR head then plays back recorded magnetic signals
(bits) which are arranged in circular tracks on the disk. In high density
disk drives bits are closely spaced linearly about each circular track. In
order for the MR head to playback the closely spaced bits the MR head has
to have high resolution. This is accomplished by close spacing between the
first and second shield layers, caused by thin first and second gap
layers, so that the MR sensor is magnetically shielded from upstream and
downstream bits with respect to the bit being read.
The MR sensor is a small stripe of conductive ferromagnetic material, such
as Permalloy (NiFe), which changes resistance in response to a magnetic
field such as magnetic flux incursions (bits) from a magnetic storage
disk. The MR sensor receives a sense current and is connected to signal
processing circuitry. When the sense current is transmitted through the MR
sensor the processing circuitry detects changes in potential which are
caused by changes in resistance of the MR sensor. The potential changes
correspond to signals received by the MR sensor. The response curve (input
vs. output) of an MR sensor has linear and non-linear portions. It is
important that the MR sensor respond along its linear portion so that the
MR sensor has a linear response. This is accomplished by magnetically
biasing the MR sensor at a biasing point on the response curve which is in
the linear response portion of the response curve.
An MR head is typically combined with an inductive write head to form a
piggyback MR head or a merged MR head. In either head the write head
includes first and second pole pieces which have a gap at a head surface
and are magnetically connected at a back gap. The difference between a
piggyback MR head and a merged MR head is that the merged MR head employs
the second shield layer of the read head as the first pole piece of the
write head. A conductive coil induces magnetic flux into the pole pieces,
the flux fringing across the gap and recording signals on a rotating disk.
The write signals written by the write head are large magnetic fields
compared to the read signals shielded by the first and second shield
layers. Thus, during the write operation a large magnetic field is applied
to one or more of the shield layers causing a dramatic rotation of the
magnetic moment of the shield layer.
Unfortunately, prior art shield layers are not stable when subjected to a
large field. Sendust (FeSiAl), which is a typical shield material, is
almost isotropic with an intrinsic uniaxial magnetic anisotropy of only
about 1 Oe. This means that magnetic domains within the Sendust material
are not well configured with respect to the MR sensor. The walls of the
domains are random and when the shield is subjected to a large applied
field, such as the write head field, the domains walls move and then
return to a different random arrangement. Accordingly, there is a change
in the stray magnetic field produced by the shield layer. Because of a
magnetostatic coupling between the MR sensor and the shield layers the
change in the stray magnetic field of the shield changes the bias point of
the MR sensor which, in turn, changes the response of the MR sensor to
signals from the rotating disk. The result is noise during the read
operation. To make matters worse Sendust typically exhibits stress induced
anisotropy due to its magnetostriction. This stress, which may be tensile
or compressive, is developed by thin film construction and/or by lapping
of an air bearing surface (ABS) at the flying surface of the head. Since
the stress induced anisotropy can easily exceed the intrinsic anisotropy
the stress induced anisotropy will control domain configuration. It could
re-orient the domain structure in undesired direction depending on the
magnitude of stress and magnetostriction.
In order for domain walls to be well configured they should be parallel to
the easy axis of the MR sensor except for a small area of closure domains
which are at each end of the shield layer. Further, the domain walls
should always return to the same wall configuration after perturbation by
a large applied field so that the magnetostatic coupling between the
shield and the MR sensor remains a constant. This can be accomplished if
the Sendust material is provided with sufficient intrinsic uniaxial
anisotropy. Another material which has been considered for shields is iron
nitride (FEN).
SUMMARY OF THE INVENTION
A multilayer magnetic structure of Fe-based (FEN, FeSiAl), NiFe and NiMn is
described for magnetoresistive head shield application. In this multilayer
structure, the use of an antiferromagnetic material, such as NiMn,
provides an exchange-coupled field for the magnetically soft Fe-based
material. The exchange-coupling field pins, i.e., fixes, the magnetic
domain structure of the Fe-based material and, therefore, provides a
magnetically stable shield material. The pinning action of the exchange
field comes from its unidirectional attribute which makes the effective
anisotropy of the shield essentially infinite as the magnetization tries
to rotate away from the easy axis in response to an applied field
perpendicular to the easy axis. For small rotations of the shield
magnetization, which is the case for disk magnetic transition detection,
the exchange field simply adds to the uniaxial anisotropy. However, for
large external fields (e.g., the magnetic field from the write head), as
the shield magnetization rotates to larger angles with the easy axis, the
effective anisotropy increases and prevents saturation of the shield.
Therefore, the original domain structure is kept intact.
The multilayer Fe-based/NiFe/NiMn structure also provides desired
properties of uniaxial anisotropy from 5 to 15 Oe, permeability greater
than 1000, coercivity along the easy axis less than 3 Oe and an exchange
field between 3 and 10 Oe so that the exchange field is greater than
coercivity.
The uniaxial anisotropy orientation provides a desirable configuration of
magnetic domains; i.e., by setting the easy axis of magnetization parallel
to the air bearing surface, domains with magnetization parallel to the
easy axis are provided over the large central portion of the shield. The
exchange field pins the domain structure against the large external field.
That is, it prevents the reconfiguration of magnetic domains in response
to large external fields, e.g., the field produced by the write head.
It is further shown that use of a NiFe buffer layer between an Fe-based
soft magnetic film and an antiferromagnetic NiMn film provides an
exchange-coupled field value which various inversely with the
magnetization times thickness value, i.e., the magnetic moment M, for the
soft magnetic film. The exchange field thus can be controlled to the
desired value.
An object of the present invention is to provide an MR read head which
operates about a constant bias point even though the shields for the MR
sensor are subjected to large magnetic fields.
Another object is to-provide a shield for an MR head which substantially
returns to a constant domain structure after being subjected to a large
magnetic field.
A further object is to provide a combined MR read head and inductive write
head wherein magneto static coupling between shield layers and an MR
sensor remains substantially constant after the shield layers are
subjected to a large magnetic field from the write head.
Still another object is to provide a shield for an MR head which has soft
magnetic properties and an intrinsic exchange field which is larger than
the coercivity along an easy axis of the shield.
Still a further object is to provide a shield of an MR head with magnetic
properties which optimize magnetic stability of the shield so that magneto
static coupling between the shield and an MR stripe is substantially
constant.
Still another object is to provide a laminated Fe-based/NiMn structure
which can be employed for magnetic head components such as shields and/or
write poles.
These and other objects and advantages of the invention will become more
apparent to one skilled in the art upon reading the description of the
invention in light of the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic illustration of a magnetic disk drive which employs
an MR read head of the present invention.
FIG. 2 is a vertical cross section of a merged MR head which employs shield
layers S1 and S2 constructed according to the present invention.
FIG. 3 is a typical response curve for an MR sensor of an MR head.
FIGS. 4A and 4B are schematic illustrations of two different domain
configurations of a shield layer with respect to an MR sensor.
FIG. 5A is a cross-sectional view of a NiFe film and corresponding
hysteresis loops along the easy axis and the hard axis.
FIG. 5B is a cross-sectional view of exchange-coupled NiFe/NiMn films and
corresponding hysteresis loops along the easy axis and the hard axis.
FIG. 6A is a cross-sectional view of a first preferred embodiment of the
present invention.
FIG. 6B is a cross-sectional view of a second preferred embodiment of the
present invention.
FIG. 7A is a cross-sectional view of a Sendust film and corresponding
hysteresis loops along the easy axis and the hard axis.
FIG. 7B is a cross-sectional view of NiMn/Sendust films and corresponding
hysteresis loops along the easy axis and the hard axis.
FIG. 8A is a lamination of Sendust, NiFe and NiMn films with their
corresponding hysteresis loops along the easy axis and the hard axis.
FIG. 8B is a lamination of NiMn, Sendust, NiFe, and NiMn with corresponding
hysteresis loops along the easy axis and the hard axis.
FIG. 9A shows a FeN film and its hysteresis loops along the easy axis and
the hard axis.
FIG. 9B shows laminated FeN and NiMn films which are separated by a buffer
layer of NiFe with corresponding hysteresis loops along the easy axis and
the hard axis.
FIG. 10 is a chart plotting the magnetic moment M of one or more soft
magnetic layers exchange-coupled to an antiferromagnet layer of NiMn
versus the unidirectional anisotropy field H.sub.UA.
FIG. 11 shows combinations of layers which are coded A and B, which code is
to be employed for illustrating various combinations of laminated shield
layers illustrated in FIGS. 12A, 12B and 12C.
FIGS. 12A, 12B and 12C show various combinations of laminated shield layers
.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings wherein like reference numerals designate
like or similar parts throughout the several views, there is illustrated
in FIG. 1 a magnetic disk drive 20. The drive 20 includes a spindle 22
which supports and rotates a magnetic disk 24. The spindle 22 is rotated
by a motor 26 which is controlled by drive controls 28. A magnetic head
30, which may be a merged MR head for recording and reading, is mounted on
a slider 32 which in turn is supported by a suspension and actuator arm
34. The suspension and actuator arm 34 positions the slider 32 so that the
magnetic head 30 is in a transducing relationship with a surface of the
magnetic disk 24. When the disk 24 is rotated by the motor 26, air is
moved by the surface of the disk, causing the slider to ride on a cushion
of air (an air bearing) slightly above the surface, in the order of 0.075
microns. The magnetic head 30 is then employed for writing information to
multiple circular tracks on the surface of the disk 24 as well as reading
information therefrom. These information signals as well as servo signals
for moving the slider to various tracks are processed by drive electronics
36.
FIG. 2 is a side cross-sectional schematic illustration of the merged MR
head 30 which includes a read head portion and a write head portion which
are lapped to an air bearing surface (ABS), the air bearing surface being
spaced from the surface of the rotating disk by the air bearing as
discussed hereinabove. The read head portion includes an MR sensor which
is sandwiched between first and second gaps layers G1 and G2 which, in
turn, are sandwiched between first and second shield layers S1 and S2. The
write head portion includes a coil layer C and insulation layer I2 which
are sandwiched between insulation layers I1 and I3 which in turn are
sandwiched between first and second pole pieces P1 and P2. A gap layer G3
is sandwiched between the first and second pole pieces at their pole tips
adjacent the ABS for providing a magnetic gap. When signal current is
conducted through the coil layer C, signal flux is induced into the first
and second pole layers P1 and P2 causing signal fringe flux across the
pole tips of the pole pieces at the ABS. This Signal fringe flux is
induced into circular tracks on the rotating disk 24, shown in FIG. 1,
during a write operation. During a read operation, recorded magnetic flux
signals on the rotating disk are induced into the MR sensor of the read
head causing a change in the resistance of the MR sensor which can be
sensed by a change in potential across the MR sensor responsive to a sense
current (not shown) conducted through the MR sensor. These changes in
potential are processed by the drive electronics 36 shown in FIG. 1. The
combined head illustrated in FIG. 2 is a merged MR head in which the
second shield layer S2 is employed as a first pole piece P1 for the
combined head. In a piggyback head (not shown) the second shield layer S2
and the first pole piece P1 are separate layers.
A typical response curve for the MR sensor, shown in FIG. 2, is illustrated
in FIG. 3. It can be seen that the MR sensor has a generally linear
response between points 40 and 42 on the response curve for a positive
applied magnetic field H. The MR sensor is biased at a point 44 selected
between the extremes 40 and 42 to provide a linear response to an applied
magnetic field, such as the magnetic data signals recorded on the disk 24.
This may be accomplished, as is well-known in the art, by an adjacent
conductor which provides a bias field magnetostatically coupled to the MR
sensor. Since the shield layers S1 and S2, shown in FIG. 2, are magnetic
layers, they are also magnetostaticatly coupled to the MR sensor. If the
magnetic properties of these shield layers S1 and S2 do not remain
constant during operation of the read head, this will affect the
magnetization of the MR head and, consequently, the position of the bias
point 44 shown in FIG. 3. This condition would seriously degrade the
performance of the read head.
FIGS. 4A and 4B illustrate how the magnetic properties of the shield layer
can affect the operation of the MR sensor. FIG. 4A illustrates a desired
magnetic domain configuration for a shield layer with respect to the MR
sensor. The primary domains 50 and 52 are elongated and parallel to the
ABS and the easy axis of the MR sensor, the domains being separated by a
domain wall 54. The magnetic spins within the domains are magnetically
oriented in opposite directions resulting in a domain wall 54. Smaller
closure domains 56 and 58 are located at opposite ends of the shield
layer. The uniform configuration of the domains illustrated in FIG. 4A is
caused by uniaxial anisotropy of the shield layer. If the shield layer was
isotropic or nearly isotropic, such as Sendust, the domains would have a
random configuration as contrasted to the uniform configuration shown in
FIG. 4A.
During a write operation of the write head shown in FIG. 2, a strong
magnetic field is produced across the pole tips of the pole pieces P1 and
P2 at the gap G3. This magnetic field, which is directed perpendicular to
the domains 50 and 52 and the easy axis of the MR sensor of FIG. 4A
dramatically reconfigures the domain structure within the shield layers.
During the write operation, the shield layers will be driven to saturation
along their hard axis, which is perpendicular to the ABS, causing each of
the shield layers to go to a single domain state with all of the spins
within the single domain aligned along the direction of the field. During
the write head operation, the read head is not operating so that these
magnetic conditions of the shield layers have no affect upon the read
head. It is what happens immediately after the write head operation that
is important to the read head operation. If each of the shield layers
returns to its original magnetic condition when the write head shuts off,
as shown in FIG. 4A, there is no change in the magneto static coupling
between the shield layers and the MR sensor. Unfortunately, this desirable
condition has not been occurring with prior art shield layers for MR read
heads.
Because of undesirable magnetic properties of prior art shield layers in MR
heads, the domain configuration can return to a metastable state, as shown
in FIG. 4B, after the write head switches to an off condition. This can
place the domain wall 54 close to or directly over the MR sensor which
causes a change in the magnetostatic coupling between the shield and the
MR sensor. This then changes the bias point 44, as shown in FIG. 3,
resulting in nonlinear or erroneous signal response by the MR sensor.
Because of imperfections in the shield layer, the domain wall 54 can
actually move after the write head switches to an off condition resulting
in what is known as Barkhausen noise. The condition shown in FIG. 4B is
the result of insufficient uniaxial anisotropy of the shield layer
allowing the domain structure to shift to a metastable state when the
write head shuts off. The present invention provides sufficient uniaxial
anisotropy and unidirectional anisotropy fields for Fe-based materials to
return the shield layer to its original domain configuration after
excitation by a large magnetic field, such as from a write head during its
operation.
FIG. 5A is a graph of hysteresis loops 60 and 62 along the easy axis and
hard axis, respectively, of a typical prior art NiFe shield layer (Ms=780
emu/cm.sup.3). The applied field H is shown along the abscissa and the
magnetic moment (M=M.sub.s t where t is the thickness of the NiFe shield
layer) of the material is shown along the ordinate. The magnetic uniaxial
anisotropy field H.sub.KU of the material is the amount of applied field H
along the hard axis of the material to obtain magnetic saturation as shown
at 64. When there is an H.sub.KU of 5.4 Oe, the domain structure will be
as shown in FIG. 4A when the applied field H is less than 5.4 Oe. This
means that the domains will be uniform and the magnetic moments of the
domains will be parallel to the easy axis of the shield material when the
applied field is zero. When the applied field H exceeds H.sub.KU
saturating the material as shown at 64, the material goes to a single
domain state with all the spins oriented in the direction of the field.
The coercivity H.sub.CE of the material is determined by an applied field
H along the easy axis. Coercivity along the easy axis is the amount of
applied field H required to commence domain wall motion.
With reference to FIGS. 6A and 6B, a multilayer magnetic structure
according to the present invention wherein a layer of soft magnetic
material is stabilized by a magnetic field provided by exchange-coupling
with an adjacent layer of antiferromagnetic material is provided for use
as magnetic shielding elements or other magnetic elements within an MR
head. FIG. 6A shows a magnetic structure 51 comprising a layer 53 of a
soft magnetic material with a layer 55 of an antiferromagnetic material in
physical contact with the soft magnetic layer. Fe alloys such as NiFe,
FeSiAl, or FeN, for example, or other suitable ferromagnetic material may
be used for the soft magnetic layer 53. NiMn, FeMn or NiO or other
suitable antiferromagnetic materials may be used for the antiferromagnetic
layer 55. In a preferred embodiment, a 30 nanometer (nm) thick NiMn film
55 is formed by sputter deposition, for example, over a 440 nm thick layer
of NiFe. As illustrated in FIG. 10, the magnitude of the exchange-coupled
field is a function of the antiferromagnetic material used, the
ferromagnetic material (i.e., the soft magnetic material) and the
thickness of the soft magnetic layer 53. Thus the amount of shift of the
BH loop (as shown in FIG. 5B) may be controlled to provide desired
operating characteristics. For example, since the magnetization, M.sub.s,
for FeN is approximately double the M.sub.s for NiFe, the same magnitude
exchange field may be obtained with NiMn and FeN as that obtained with
NiMn and NiFe-having a NiFe layer twice as thick as the FeN layer
thickness.
FIG. 6B shows a second embodiment of the magnetic structure according to
the present invention wherein a thin buffer layer of a second soft
magnetic material is inserted between the antiferromagnetic layer and the
first soft magnetic layer for example, a preferred embodiment of the
multi-layer structure 57 comprises a layer 59 of a first soft magnetic
material, such as FeSiAl, a buffer layer of a second soft magnetic
material, such as NiFe, and a layer 63 of an antiferromagnetic material
such as NiMn. In this case, the magnitude of the exchange-coupled field is
proportional to the sum of the thicknesses of the first soft magnetic
layer 59 and the buffer layer 61.
By depositing a 30 nm thick NiMn film on top of a 440 nm thick NiFe film
and annealing the bilayer films for 10 hours at 320.degree. C., the
easy-axis hysteresis loop can be shifted by an amount of 4.2 Oe, as shown
in FIG. 5B. This shift, defined as a unidirectional anisotropy field
(H.sub.UA), results from exchange coupling between the NiFe and NiMn
films. Coercivity is one half of the applied field H between the vertical
lines of the loop 66 intersecting the abscissa. Since the hysteresis loop
66 has been translated to the right by H.sub.UA, the amount of applied
field required to commence a domain wall motion is the sum of the H.sub.UA
plus the coercivity H.sub.CE. The exchange field H.sub.UA increases the
uniaxial anisotropy to stabilize the uniform domain configuration, as
shown in FIG. 4A. A certain degree of stability is desirable so that a
uniform domain configuration is established and a return to this
configuration is assured upon relaxation of an external applied field H.
If the uniaxial anisotropy and/or exchange field is too large, however,
the magnetic moments become too large and the shield material loses
permeability. The hysteresis loop along the hard axis for the NiFe/NiMn is
shown at 68 in FIG. 5B.
A good shield material requires high permeability. This means that the
shield material has soft magnetic properties and will guide magnetic flux
such as external applied fields H thus shielding the MR sensor. We have
found that the most desirable magnetic properties for a shield layer of a
read head are as follows: uniaxial anisotropy H.sub.KU greater than 5 Oe
and less than 15 Oe; permeability .mu. greater than 1,000; coercivity
along the easy axis H.sub.c less than 3 Oe; and exchange field H.sub.UA
greater than 3 Oe and less than 10 Oe provided H.sub.UA is greater than
H.sub.C, and H.sub.KU plus H.sub.UA equals 5 to 15 Oe.
Assuming an external field H is applied perpendicular to the easy axis of a
shield layer which has uniaxial anisotropy and exchange fields, the
magnetic moment M of the shield material assuming an angle .phi. with
respect to the easy axis due to the external field, the following
equations will apply:
##EQU1##
minimizing this energy gives:
##EQU2##
For large external fields, angle .phi. becomes large and the material
becomes more stable due to the term H.sub.UA /Cos.phi.. Accordingly, upon
removal of the external field H, the magnetization is forced to return to
a uniform state parallel to the easy axis.
For small .phi., sin .phi.=tan .phi., then
H=(H.sub.KU +H.sub.UA) sin .phi.
where
H.sub.KU =uniaxial anisotropy field
H.sub.UA =unidirectional anisotropy field.
Sendust (FeSiAl), which has M.sub.s =1022 emu/cm.sup.3, is a typical shield
material for read heads because of its soft magnetic properties, high
hardness and high resistance to corrosion. A single Sendust layer is shown
in FIG. 7A. Unfortunately, this Sendust layer after annealing for 2 hours
at 500.degree. C. has low uniaxial anisotropy H.sub.KU which makes it
nearly isotropic. This can be seen from the graph shown in FIG. 7A. FIG.
7A shows hysteresis loops 70 and 72 along the easy axis and hard axis of
the material. The uniaxial anisotropy H.sub.KU is 3.6 Oe. When this single
layer of Sendust is subjected to an on and off external field H, the
domain walls will change and come back to a different condition than that
shown in FIG. 7A. Because of a change in the domain configuration between
read operations of the read head, the MR sensor of the read head is
subjected to different magnetostatic relationships with the first and
second shields S1 and S2 of FIG. 2 causing a change in the bias point 44
shown in FIG. 3 and erroneous or nonlinear response of the read head.
We have found that by depositing a layer of Sendust on top of a layer of
NiMn that the magnetic properties of the Sendust can be significantly
improved to maintain a constant magnetostatic relationship between the
shield layers and the MR sensor after excitation by an external magnetic
field H. As shown in FIG. 7B, we have deposited 380 nm Sendust on top of a
30 nm thick NiMn film. The resulting hysteresis curves 80 and 82 along the
easy axis and hard axis, respectively, of the lamination is shown in FIG.
7B. The resulting uniaxial anisotropy H.sub.KU has been increased to 5.7
Oe. This level exceeds the stress-induced anisotropy so as to maintain a
uniform configuration of the domains within the material as shown in FIG.
4A. The coercivity H.sub.CE along the easy axis is 2.2 Oe. The uniaxial
anisotropy is sufficient to return the domains within the material to a
constant configuration after excitation by an external field.
In another embodiment we have modified the embodiment shown in FIG. 7B by
providing a buffer layer of NiFe between the Sendust and NiMn as shown in
FIG. 8A. This produced a surprising result by providing an exchange field
of 2.5 Oe after annealing for 10 hours at 320.degree. C. The exchange
field is evident from the fact that the easy axis 84 has been translated
to the right along the abscissa, the hysteresis loop along the hard axis
being shown at 85. With this lamination the uniaxial anisotropy H.sub.KU
was 5.0 Oe. FIG. 8B shows still a further embodiment over the FIG. 8A
embodiment by sandwiching the Sendust layer between the buffer layer and
another NiMn layer. Another surprising result was obtained by significant
increases in the uniaxial anisotropy H.sub.KU and the exchange field
anisotropy H.sub.UA. In FIG. 8B the hysteresis loop along the easy axis is
shown at 86 and the hysteresis for the hard axis is shown at 87. In
comparing the graph in FIG. 8B with the graph in FIG. 8A, the uniaxial
anisotropy H.sub.KU has been increased from 5.0 Oe to 9.5 Oe and the
exchange field anisotropy has been increased from 2.5 Oe to 3.4 Oe. Both
of the Sendust lamination embodiments shown in FIGS. 8A and 8B provide
highly stable shield material which is assured of returning to a uniform
domain state at the application of high external fields, such as from the
write head of a merged MR head. Between the two embodiments, 8A and 8B,
the embodiment shown in 8B is significantly enhanced over the 8A
embodiment.
Another material which exhibits soft magnetic and good mechanical
properties for shields of an MR head is FeN (Ms=1552 emu/cm.sup.3). A
single layer of FeN with a thickness of 244 nm is illustrated in FIG. 9A.
Hysteresis loops 90 and 92 along the easy and hard axis, respectively, of
the FeN layer is also shown in FIG. 9A. The FeN layer exhibited uniaxial
anisotropy H.sub.KU of 3.3 Oe and a coercivity H.sub.CE of 2.7 Oe. We have
discovered that by laminating an FeN layer with a NiMn layer and annealing
for 10 hours at 320.degree. C., we can achieve an exchange coupling which
will establish an exchange field. In FIG. 9B we have deposited NiFe (10
nm)/NiMn (30 nm) films on top of 236 nm FeN. The resulting hysteresis
loops 100 and 102 along the easy axis and the hard axis, respectively, are
also shown in FIG. 9B. After annealing for 10 hours at 320.degree. C., the
uniaxial anisotropy H.sub.KU was 6.4 Oe, the coercivity H.sub.CE was 5.8
Oe and the exchange field H.sub.UA was 4.8 Oe. It can be seen from FIG. 9B
that the hysteresis loop 102 is symmetrical about the abscissa and the
ordinate while the hysteresis loop 100 along the easy axis is translated
to the right along the abscissa. As stated hereinabove, the exchange field
increases the uniaxial anisotropy of the FeN layer which promotes return
of the domains within the FeN to the same configuration after excitation
by an external field H.
FIG. 10 is a chart illustrating magnetic moment of one or more soft
magnetic layers versus H.sub.UA. The filled circles are four examples of a
lamination of NiFe and NiMn at various thicknesses of the NiFe. It can be
seen that there is a linear relationship between the thickness of the NiFe
and the exchange field H.sub.UA with the exchange field H.sub.UA
increasing as the thickness of the NiFe layer decreases. Another example
is shown by the open circle which is a lamination of FeN and NiMn with a
buffer layer of NiFe therebetween. The thickness here is the combined soft
magnetic layers of FeN and NiFe. It can be seen that the exchange field
H.sub.UA is very close to the filled circle for the NiFe and NiMn
lamination. The same holds true for the open square which is a lamination
of NiMn, Sendust, NiFe, and NiMn. This chart shows that the controlling
factor for tailoring the exchange field H.sub.UA is the use of buffer
layer of NiFe and thickness of shield material.
FIG. 11 is a schematic illustration of various combinations of layers coded
A and B to be employed for various embodiments illustrated in FIGS. 12A,
12B and 12C. The only difference between the FIG. 12A and FIG. 12B
embodiments is that in FIG. 12A the NiMn layer is deposited on the
Fe-based layer and in FIG. 12B the Fe-based layer is deposited on the NiMn
layer. In the FIG. 12C embodiment, the Fe-based layer is sandwiched
between NiMn layers. The advantage of this embodiment is that we can
control the Fe-based layer thickness for desired H.sub.UA field.
It has been found that Ni.sub.81 Fe.sub.19 buffer layers with a thickness
in the range 20-200 .ANG. work for purposes of the present invention. Each
of the Fe-based layers can be in the range 0.05-1 .mu.m, with total
thickness of the soft material about 1 .mu.m, the Fe-based layers being
FeSiAl (Sendust), FeN and NiFe.
It should be understood that because of the unique properties of the
aforementioned laminations that they can be used for other applications,
such as write poles for an inductive head. With these materials the write
poles will be assured of returning to a stable domain structure after each
write operation.
Although the invention has been described in terms of the specific
embodiments, the inventors contemplate modifications and substitution to
various components of the invention which would occur to a person of
ordinary skill in the art, and therefore, would be in the scope of the
invention, which is to be limited only by the claims which follow.
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